Since Newton's dispersion of light into a continuous color “spectrum,” spectroscopy has been viewed primarily as a frequency-based technique. Bunsen, Foucault, Kirchhoff, and many others identified unique spectral lines for elements and compounds based on the emission and absorption of radiation at various frequencies. Spectroscopy has historically been used to obtain a wide range of information about nuclear, atomic, and molecular properties.
Early on, the determination of spectral lines, or energy levels, helped elucidate the principles of quantum mechanics through studies of the hydrogen atom and provided a means for testing atomic theory. Since then, several spectroscopy techniques to determine absolute transition frequencies (or, equivalently, wavelengths) have been developed, involving the emission, absorption, or scattering (e.g., Raman) of radiation. The series of spectral lines of hydrogen are named for Balmer and Rydberg, who observed them within and beyond the visible wavelengths. As previously mentioned, such frequency-dependent absorption and emission spectroscopy played a fundamental role in the development of quantum mechanics and the “new” atomic theory by identifying discrete energy levels.
With the invention of coherent high-intensity radiation sources at microwave (maser) and optical (laser) frequencies, with tunable, narrow spectral line-widths, targeted absorption spectroscopy of atoms and molecules with high frequency resolution is provided. The advent of tunable, coherent radiation sources at microwave and optical frequencies led to the age of modern atomic spectroscopy, where a primary approach is to identify absorption spectra of natural and artificial atoms and molecules as the source frequency v is varied to fulfill the resonance conditions ΔE=hv, where ΔE is the energy-level separation and h is Planck's constant. The technique is now commonplacein research labs and usually involves shining a beam of light on a sample and watching how it absorbs light as the frequency of the radiation is swept through a range of values. An atom, for example, absorbs radiation at a specific set of frequencies that correspond to gaps between the energy levels of its electrons.
Spectroscopy has traditionally been viewed as a frequency-based measurement technique. Frequency-dependent absorption and emission spectroscopy has long played a fundamental role in the characterization of quantum systems. As previously mentioned, the development of coherent microwave (maser) and optical (laser) sources, high-intensity radiation with tunable, narrow spectral line-width, has further enabled targeted absorption spectroscopy of atoms and molecules with high frequency resolution. However, the application of broadband frequency spectroscopy is not universally straightforward. This is particularly relevant for certain classes of multi-level quantum systems (including, but not limited to, natural and artificial atoms, molecules, defects, impurities, which assume quantized energy levels that extend into microwave, millimeter wave and terahertz regimes. Although certainly not an impossible task, a broadband frequency-based spectroscopic study of such multilevel quantum systems in excess of around 50 GHz, becomes extremely challenging and expensive to implement due to numerous frequency-dependent effects (e.g., frequency dispersion and the requisite tolerances to control impedance), and due to the general requirement of multipliers that are inefficient and intrinsically noisy.
The abovementioned difficulty has been problematic for researchers performing studies on multilevel quantum systems, such as, for example, artificial atoms. Artificial atoms exhibit properties of ordinary atoms, including discrete energy levels. Such atoms could potentially be used to store and process data. Unfortunately, the problem in using artificial atoms as putative quantum-information systems is that the gaps between the levels tend to be in the problematic millimeter and microwave region.
Thus, a heretofore unaddressed need exists in the industry to address the aforementioned deficiencies and inadequacies.